1. Field of the Invention
The present invention relates to a rotary servovalve for use in a power steering device.
2. Description of the Prior Art
Rotary servovalves for use in power steering devices are required to have two-step valving characteristics as shown in FIG. 6 of the accompanying drawings. According to the two-step valving characteristic curve, as a steering wheel is turned, the steering wheel is not hydraulically power-assisted in a region when a steering wheel torque T.sub.M is small, is gradually hydraulically power-assisted when the steering wheel torque T.sub.M exceeds a point a, and is increasingly hydraulically power-assisted when the steering wheel torque T.sub.M is further increased beyond a point b. Therefore, the steering wheel is hydraulically power-assisted in two steps.
To gain the above two-step valving characteristics, the conventional rotary servovalve includes a valve land which generally has a small chamfer. In most of prior rotary servovalves, such a chamfer is of a simply inclined shape for practical machining reasons. With the prior chamfer configuration, however, there is a limitation on the selectibility of valving characteristics in the hydraulically power-assisted regions, and hence it is difficult to provide various steering characteristics to meet different kinds or grades of vehicles. Particularly, the simply inclined chamfer fails to give a clear characteristic change at the intersection between the first and second power-assisted regions in the two-step valving characteristics.